Field of the Invention
The present invention relates to inks based on layered materials, methods of manufacture of such inks, methods of printing using such inks, and the resultant printed materials and devices. The invention is of particular, but not exclusive, application to functional (e.g. conductive, insulating) and/or light-transmissive printed material and/or inks.
Related Art
Flexible electronics is a rapidly expanding research area. Applications include touch screens, electronic paper (e-paper), sensors, radio frequency tags, photovoltaic cells, light-emitting diodes and electronic textiles. To date, it mainly relies on two fabrication strategies: one in which substrates bearing field-effect transistors (FETs) are bonded to plastic by transfer printing, or pick-and-place methods; another in which FETs are prepared directly on the target substrate by several coating, curing, and lithographic steps. Techniques such as rubber stamping, embossing, roll to roll processes such as screen, inkjet, gravure and flexo printing, web or slot die coating reduce the number of such fabrication steps.
Roll to roll printing or coating processes are promising techniques for large-area fabrication of flexible plastic electronics. A range of components can be printed, such as transistors, photovoltaic devices, organic light-emitting diodes (OLEDs), and displays. Roll to roll printing processes are versatile, involve a limited number of process steps, are amenable for mass production, and can deposit controlled amounts of material. In particular drop-on-demand inkjet printing has progressed from printing text and graphics to a tool for rapid manufacturing, being now a well-established technique to print thin-film transistors (TFTs) based on organic conducting and semiconducting inks. However, their mobilities, μ<0.5 cm2V−1 s−1 are still much lower than standard silicon technology. Several approaches aim to improve these results, such as the use of polysilicon, zinc oxide nanoparticles and carbon nanotubes (CNTs). Metal nanoparticle inks are considered not to be stable in ordinary solvents, such as deionized (DI) water, acetone, isopropyl alcohol, N-methylpyrrolidone (NMP), or tetrahydrofuran [References Singh et al (2010) and Luechinger et al (2008)]. Therefore they need to be chemically modified in order to be dispersed via the use of stabilizers, which usually degrade in a couple of years. Metal nanoparticles also tend to oxidize after the printing process [Reference Singh et al (2010)]. Inkjet printed CNT-TFTs have been reported with μ up to 50 cm2V−1 s−1 and an ON/OFF ratio of about 103 [Reference Ha et al (2010)].
Graphene is the two-dimensional (2d) building block for sp2 carbon allotropes. Near-ballistic transport and high mobility make it an ideal material for nanoelectronics, especially for high frequency applications. Furthermore, its optical and mechanical properties are ideal for micro- and nanomechanical systems, thin-film transistors, transparent and conductive composites and electrodes, and photonics. A review of graphene photonics and optoelectronics is set out in Reference Bonaccorso et al. (2010).
It is known that graphene can be isolated by micromechanical exfoliation of graphite [Reference Novoselov et al (2005)]. This technique gives good results in terms of purity, defects, mobility, and optoelectronic properties. However, large scale production approaches are needed for widespread application. Attempts have been made to provide large-scale production methods by chemical vapour deposition (CVD) [Reference Li et al. (2009)], sublimation of Si atoms by heat treatment of silicon carbide [Reference Berger et al. (2004)], segregation from metal substrates and liquid phase exfoliation (LPE) [References Hernandez et al (2008), Lotya, et al (2009), Valles et al (2008) and Hasan et al (2010)]. Among these, the present inventors consider that LPE is the best candidate for producing printable inks.
Graphite can be exfoliated by chemical wet dispersion followed by ultrasonication, both in aqueous and nonaqueous solvents. Dispersions can be achieved by mild sonication of graphite in water with dispersants (e.g. surfactants, polymers etc), followed by sedimentation based ultracentrifugation [References Hernandez et al. (2008), Hasan et al (2010) and Marago et al (2010)]. In particular, bile salt surfactants are reported to allow the isolation of flakes with controlled thickness, when combined with density gradient ultracentrifugation [Green and Hersam (2009)]. Exfoliation of graphite-intercalated compounds and expandable graphite has also been reported.
LPE was first achieved through sonication of graphite oxide, following the Hummers method [Reference Hummers and Offeman (1958)]. The oxidation of graphite in the presence of acids and oxidants disrupts the sp2 network and introduces hydroxyl or epoxide groups, with carboxylic or carbonyl groups attached to the edge. These make graphene oxide (GO) sheets readily dispersible in water and several other solvents. Although large GO flakes can be produced, these are intrinsically defective and electrically insulating. Despite attempts by several workers, reduced GO (RGO) does not fully regain the pristine graphene properties, including electrical conductivity. It is thus important to distinguish between dispersion-processed graphene flakes retaining the electronic properties of graphene, and insulating GO dispersions. Several groups have reported GO-based inks. Reference Dua et al (2010) reported inkjet printed RGO films for sensor applications, while Reference Luechinger et al (2008) produced RGO-stabilized Cu nanoparticles as low temperature metal colloids, to replace standard metal nanoparticle inks, which require high-temperature sintering post-processing. Mobilities up to 90 cm2V−1 s−1 have been achieved for highly reduced GO films by inkjet printing [Reference Wang et al (2009)], with an ON/OFF ratio up to 10.
US 2010/0000441 discloses a conductive ink based on nano graphene platelets. The nano graphene platelets are formed by dispersing graphite in a liquid medium such as water, alcohol or acetone, adding a dispersing agent or surfactant and subjecting the suspension to direct ultrasonication. The ink was used for printing using an inkjet printer. A resistivity for a single print layer of as low as 75 kΩ/square was measured.
US 2008/0279756 provides a similar disclosure to US 2010/0000441, but additionally suggests the processing of other layered materials than graphite, such as transition metal dichalcogenides.